In the plastics industry, as well as in other industries, industrial processes performed by machines are typically controlled by programmable logic controller (PLC)-based control systems. These PLC-based control systems are typically comprised of a plurality of input/output (I/O) devices integrated with the machine performing the process, a PLC, one or more specialized I/O control boards, and a general purpose digital processor. The PLC stores and executes a control program that causes the PLC to issue commands to the I/O devices thereby controlling the operational sequence of the machine process. Some of the commands issued by the PLC are issued through the specialized I/O control boards. The general purpose digital computer provides an interface between a human operator and the control program being executed by the PLC. The general purpose digital computer may receive process parameters and other information from the PLC and present such information for viewing by the human operator. Similarly, the human operator may use I/O devices (such as a keyboard or a mouse) associated with the general purpose digital computer to input process parameters and other information relating to the controlled process into the PLC. This type of PLC-based process control system is disclosed in U.S. Pat. No. 5,316,707, U.S. Pat. No. 5,518,671 and U.S. Pat. No. 5,062,052.
One drawback of PLC-based control systems is the relatively limited processing power of PLCs. PLCs are clocked at relatively low speeds. This limited processing power of PLCs may prevent a PLC controlled machine from performing a process at optimal speeds and/or at optimal quality and efficiency.
The limited processing power of PLCs may also require that the monitoring and control of some I/O devices on the machine be performed by additional and specialized I/O control boards. For example, in the plastic injection molding process, certain process temperatures are particularly important and sensitive parameters of the process. Relatively small changes in temperature in relatively small periods of time must be detected and appropriate control commands issued on a real-time basis. However, a typical PLC cannot perform this function at the same time that the PLC is performing other functions. Thus, a specialized I/O control board that operates as a real-time signal processor must be operably connected between the temperature I/O device and the PLC. The addition of this specialized I/O control board to the PLC-based control system increases the cost of the system and the chances for system failure.
The limited processing power of PLCs also makes them an ineffective means for providing a human operator interface to the control program. Rather, PLC-based process control systems require a general purpose digital computer operably connected to the PLC for this purpose. This dual hardware requirement may result in relatively large cost expenditures associated with the purchase, installation and maintenance of PLC-based process control systems. Utilizing both of these hardware components in a process control system also increases the chances for system failure.
Yet another drawback of PLC-based control systems is that the PLC technology is usually proprietary technology of the process control system manufacturer. Consequently, with respect to technology upgrades, operators and other end-users of the PLC-based control systems are at the mercy of process control system manufacturers.
Yet another drawback of PLC-based control systems is that the process control program is typically written in a relatively complex low-level language, such as ladder logic. However, purchasers and other end users of process control systems are rarely trained in the art of low-level programming. Thus, when a purchaser or other end user of a process control system wishes to write a new process control program or to modify an existing process control program, a trained low-level language programmer must usually be retained or consulted. However, given the complexity of low-level languages and the fact that a typical low-level language command has only a basic, minimal functionality within a program, even a trained programmer may be required to write and/or edit large amounts of control program code to make even minor modifications to the control program. This may result in significant cost expenditures as well as significant downtime for the process control system and the machine.
Yet another drawback of PLC-based control systems is that, due to the proprietary nature of such systems, PLC-based control systems cannot be operably connected to conventional computer networks.
Thus there exists a need in the art for process control systems comprising fewer hardware components and greater processing power. Furthermore, there exists a need in the art for readily upgradeable process control systems that may be operably connected to conventional computer networks. Furthermore, there exists a need in the art for process control systems that may be relatively simply and intuitively programmed by individuals unfamiliar with low-level programming languages (e.g., purchasers and other end users of process control systems).